1. Field of the Invention
The present invention relates to a printing medium suitable for use in forming images with water-based inks, and an image forming process, and particularly to a printing medium suitable for use in multi-color printing using water-based inks containing a water-soluble dye and an image forming process using this medium.
The present invention also relates to a coating composition suitable for use in producing such a printing medium, and an image forming process using the medium.
2. Related Background Art
With the improvement in performance of ink-jet printing apparatus, such as speeding up of printing and multi-coloring of images, in recent years, printing media have also been required to have higher and wider properties. More specifically, they are required to simultaneously satisfy, for example, the following properties:
(1) having high ink absorptivity (ink absorbing capacity being great, and ink absorbing time being short); PA1 (2) providing printed dots high in optical density and clear in periphery; PA1 (3) providing printed dots having a substantially round shape and a smooth periphery; PA1 (4) being able to stably store images formed thereon for a long period of time without deterioration of quality (in particular, under conditions of a high-temperature and high-humidity environment); PA1 (5) undergoing little changes in the properties even at varied temperatures and humidities and no curling; PA1 (6) undergoing no blocking; and PA1 (7) providing prints the images of which do not fade even when the prints are posted over a long period of time. PA1 (1) since inks are absorbed in voids defined by silica, their volume is limited by the void volume; PA1 (2) the resulting medium cannot be used as a printing medium of which transparency is required; PA1 (3) the resulting medium has no dye-fixing property, and causes migration of dyes in a high-humidity environment; PA1 (4) it is difficult to constitute a printing medium having such high ink absorbency and water resistance that it is applicable to any high-speed color ink-jet printer. PA1 (5) when the resulting printing medium is required to have transparency or high gloss, it is necessary to use an ink-absorbent material scarcely undergoing light scattering; and PA1 (6) since some inorganic pigment may have a catalytic action on the deterioration reaction of dyes, it is desirable to use an ink-absorbent material by which the deterioration of the dyes is prevented. PA1 (1) A polyepoxide having at least three epoxy groups is subjected to a quaternizing reaction with at least one amine and to a reaction for adding at least two acrylic esters; PA1 (2) An acrylic monomer having a tertiary amino group, such as dimethylaminoethyl (meth)acrylate, is added to a polyepoxide having at least three epoxy groups; and PA1 (3) At least one acryl group of a polyacryloyl compound having at least three acryloyl groups is subjected to an addition reaction with trimethylamine or the like into a cationized product. PA1 (a) Polyvinyl alcohol reacted with an acid anhydride such as maleic anhydride, succinic anhydride or phthalic anhydride, and an epoxy resin; PA1 (b) An acrylic resin obtained by copolymerizing a glycidyl (meth)acrylate, and an acid anhydride; PA1 (c) An alcohol-soluble and water-insoluble acrylic resin in which a water-soluble high-molecular compound having crosslinkable alkanol groups in its molecule is used in combination with any one of N-methylol(meth)acrylamide and N-butoxymethyl(meth)acrylamide; PA1 (d) An amino type acrylic resin obtained by copolymerizing acrylic monomers having a primary, secondary or tertiary amino group in its side chain; PA1 (e) A resin composition containing a polymer of ethylene oxide having at least two hydroxyl groups in its molecule and a blocked isocyanate; and PA1 (f) A methylolized polyamide resin or a methylolized polyamide resin grafted with acrylamide, and the like.
As printing media used in such ink-jet printing systems, for example, Japanese Patent Application Laid-Open No. 59-35977 describes exclusive coated paper provided with a coating layer containing a pigment.
However, general paper for ink-jet including such coated paper ensures its ink absorbency by using a great amount of an inorganic porous pigment. When inorganic particles typified by, for example, silica are used, the following problems are offered. That is,
By the way, it has been known to use water-absorbent fine particles composed of an organic polymer in printing media for ink-jet printing. For example, Japanese Patent Application Laid-Open No. 2-223466 discloses a printing medium provided with an ink-receiving layer containing high water-absorbent resin particles and a binder, while Japanese Patent Application Laid-Open No. 7-61114 discloses a printing sheet comprising water-absorbent gel particles. Further, Japanese Patent Application Laid-Open No. 7-228039 discloses an ink-receiving layer composed of partially crosslinked multi-layer fine particles and a binder.
The resin particles disclosed in Japanese Patent Application Laid-Open No. 2-223466 have a feature that they can impart higher ink absorbency compared with inorganic particles. Since the resin particles have such high water absorbency that they can absorb water 50 to 1,000 times as much as their own weight, however, the resulting printing medium tends to cause deterioration in the resolution of images and is liable to lower the strength of its portions on which images have been formed.
In the conventional typical water-absorbent particles, their rate of water absorption and water retentivity are adjusted by crosslinking polymer chains having carboxyl groups.
Since most of dyes used in inks for an ink-jet system have a sulfonic group and/or a carboxyl group like acid dyes and direct dyes, the dye molecules diffuse together with solvents for inks after printing when the conventional water-absorbent particles having carboxyl groups are used in a printing medium, so that such a printing medium is difficult to be colored. Besides, since the printing medium is easy to exude the dyes by application of water, it has been necessary to improve a color-fixing property and a water fastness of images to be formed.
Further, the conventional resin particles absorb water in excess, so that coating films formed therefrom tend to lower their strength. The films also involve a problem that since they tend to cause tacking, the resulting prints are poor in storability.